Television system



May 28, 1940- F. scHRTER TELEVISION SYSTEM Filed Aug. '7, 1937 2Sheets-Sheet 1 w QW INVENTR' F/Q/TZ SCHROTER BY f? ATTORNEY May 28,1940.F. scHRTER TELEVISION SYSTEM Filed Aug. f7, 19357 2 Sheets-Sheet 2 IN VE NTO R ,fR/rz SCH/zov-ER A AV J l-uw- ATTORNEY Patented May 28, 1940UNITED vSTATES y 2,202,605 TELEVISION SYSTEM Fritz Schrter, Berlin,Germany, assigner to Telefunken Gesellschaft fr Drahtlose Telegraphie m.b. H., Berlin, Germany, a corporation oi Germany Application August 7,1937, serial No. 157,818 l In Germany August 29, 1936 5 Claims. (Cl.178-68) My invention relates broadly to the transmission of opticalimagesand moreparticularly to a method of and apparatus for transmittingthese images by television.

In the usual picture transmission method, more particularly inconnection with television work, it is the absolute brightness orshading values of the constituent picture elements` that are transmittedto a remote point even in cases where they have not changed as comparedwith the values of the corresponding picture element in the:transmission of the preceding frame in moving picture transmission. Thefact that this means anA unnecessary load on the transmission channel,both from the View point of the intensity of the signals to betransmitted as well as also, possibly, from the viewpoint of therequired frequency band should be appreciated.

Therefore, according to the present invention the. brightness or shadingor densityvvalues of the picture elements are transmitted only to theextent where, contrasted with the corresponding and respective pictureelements of the preceding frame, they have experienced an actual change.But in accordance with the basic idea, namely, of transmitting onlydifferences or diierent values, that is to say, to transmit to a distantpoint, as it were, only one picture which could be designated as adifference picture, 'rather than the entire picture, there are`conceivably quite a number of different embodiments.

Among the letter there is an embodiment vml which transmisison occurs ata velocity independent of the picture content and where, for instance,if the picture is dissected or scanned in straight parallel lines orstrips, one -works with a constant line and frame frequency. Among.

the methods of transmitting` pictures at a velocity independent of thepicture content may be regarded to fall also a spiral scanning methodwherein the velocity of the scanning pencil is constant or wherein thevelocity insidev the course or time of scanning each individual frame issubject to variation according to the same law. A transmission to aremote point of a difference picture which proceeds at a rate of a'velocity which, in the sensefas hereinbefore outlinedis independent o fthe picture content, oiers the merit and advantage over the transmissionmethod involving-always the absolute values of picture elementbrightness or shading as heretofore disclosed that the transmissionchannel, that is to say, for instance, an RF .carrier wave will becalled upon to carry' smaller ,amplitudes `and that as a resultinterference of neighboring transmitters will be diminished.

But within the scope of the basic idea of the invention the diierentialpicture could be transmitted also at a rate of speed which is dependentuponV the comparative brightness values. This is feasible, forinstancaby what is known in the art as velocity modulation. In thismethod, the scanning beam or pencil is so acted upon in reference to itsyspeed that at the receiving end, the intensity of the pencilre-creating and delineating the image may stay constant and stable,while, on the contrary, the speedv of its migration or sweep over thearea on which the incomingv image is to be re-createdwill be so muchsmaller the brighter any particular spot or element yof the picture istobe re-created. If the differential picture is transmitted by thisprinciple, then the scanning pencil or beam will have to sweep all thoseportions of the dilerence picture at the sending end at a high rate ofspeed at which no change has occurred, while the delineating speed willbe slower in the case of such picture elements where the differencepicture or frame has a finite brightness or shading value.

However, inasmuch as the major part of Ithe elements of a televisionimage is'altered from one frame to the next (imagining, for example, a

rscene in which a limited number of persons appear on a very variegatedand detailed `background) the transmission of the difference picture,for the same high deiinition, is accomplishable inside a briefer periodof time than when using the method known in the earlier art in whichAagain and again, the absolute shading values of the picture elementswere handled andr transmitted rather than` the differential values.However, fundamentally speaking this means that the width of therequisite transmission channel is reduced without this being associatedwith a 4 loss of picture detail ashas heretofore been true.- Thisprogress could be utilized in practice either in the manner that for thetransmission of the video signals a channel of reduced width is used orelse that the picture so sent is made of higher deiinition and richer indetail, in other words, by accommodating inside a given yframer area agreaternumber of picture elements without the necessity of enlarging thecorresponding transmission channel. 4

My invention will best be understood by reference to the drawings inwhich Fig.1 is an embodiment of my invention in which the differentialpicture. is transmitted at a velocity independent of the picturecontent,

Fig. 2 is a receiving arrangement, and

Fig. 3 is a schematic arrangement.

Referring to Fig. l, I0 denotes a strip of lm bearing frames II throughI4. Anteriorly of the said lm strip are two devices I5 and I6, say, twoBraun type or cathode ray tubes which for each of the film frames I2vand I3 produce a spot designed to scan the two frames of the film insynchronism and in phase. Posteriorly of the film strip are mounted twophoto-electric cells I1 and I8 which are provided with a common positivepotential source of supply I9 and whose currents flow through twodistinct resistances 2li,`

2I. These resistances 26 and 2l in the manner as shown in the drawingare united with the control grids of two tubes 22 and 23, the gridcircuits thereof including also'the biasing voltage sources 24 and 25.The plate circuits of the two tubes contain the resistances 26 and 21and distinct plate potential sources of supply 28, 29. Two cathode raydevices 36 and 3l have so-called storing surfaces 32 and 33,respectively are unbroken metal sheets (so-called mosaic elements).which latter are insulated from each other as well as from the jointco-operating coat so that in reference to this coat they represent tinycondensers.

j and 23, respectively. On the right-hand side of the storing surface 32and 33, respectively, is a device known from the cathode ray picturescanning art and which, respectively, consistsy also of secondaryemission anodes 44, 45, a pair'of deflector plates 46, 41, vanodes 48,43, cathodes 50,

5I, and also control grids 52, 53, respectively.

r Connected with therdeflection plate pairs 36, 31

(which, however, could also be replaced by dev fiection coils) is asawtooth wave generator 54 Y, which'furnishes deflection potentials forthe two [pairs of plates producing deections in the two 'co-ordinatesenses of the picture. "ing surfaces 32 and 33 and the anodes 44 and 45,

From the storleads are'brought respectivelyto amplifiers 55 and 56,lboth of which feed an RF transmitter 51.

Also for the pairs of dei-lector plates 46, 41, which could yalso bereplaced by deflector coils, there is provided va joint saw-tooth wavegenerator 58..

The control grids 52, 53 of the two storing dej vices are connected witha device 59 consisting, for example, of a so-called multivibratorAsupplying positive and negative impulses of like duration and whichfurnishes the grids 52 and 53 alj 14 shown only schematically with thebusbars 15;v 16 positioned perpendicularly in the picture proternatelywith a voltage designed to block out the cathode ray beam.

An arrangement of the kind shown in Fig. 1 f operates in the manner thatupon the two'pho- :Vavtoelectrc cells I1 and I8 luminous uxesycorresponding to the absolute brightness or shading values of twocongruous picture elements in the two frames I2 and I3 are made toimpinge. In-

" asmuch as the drops of voltage across the re- Y' inthe two tubes 22and 23, it will be'seen vthat the'potential of the control grid of tube22 will be "shifted in the positivel sense only if the respectivepicture element of the lm frame I3 tube 23.

Vof supply 24 and 25 to suit this scheme is to l0 68 connectionisestablished with two'switch-l ycorresponding to the' various picturepoints may lconsist, for instance, of small-sized.V cathode ray 'lampsI1 and 18 having'ajcontrol grid, with a condensor suchas` 19er 80,respectively,';being "interposed betweenthe control grid. and the..

cathode of each lamp.- It willbe understood that 70 happens to be darkerthan the corresponding picture point of the lm frame I2. In this casethe potential of the control grid of the tube 23 shifts into thenegative region seeing that the fall of potential across the-resistance26 which furnishes 5 for the tube 23a negative grid voltage prevailsover the fall of potential across the resistance 2| which represents apositive grid voltage for the The size of the biasing potential sourcesbe chosen in such a way that, in the presence of the above assumptions,only'tube 22 will conduct current, while tube 23 is blocked. But if,conversely, the picture element under consideration. in lm frarneMI'Z isbrighter than the respective 15 picture element in frame I2, then thedrop of potential across resistance 2| willsurpass that acrossresistance 26 and tube 22 stays blocked, while rtube 23 will conductcurrent. Now. the cathode ray pencils issuing from cathodes 46 and 204I, by theaid of `control grids 42 and 43, re-

spectively, willbe acted upon and modulatedin accordance with thedifferential valuesof they picture shading or brightness vvalues, inother words, in accordance withthe brightness of the differentialpicture in such a way that the elementary capacities of the storingsurfaces "32 and 331 overk which the modulated cathode-'ray beamis sweptby the aid of the pairs of deector plates 36 and 31 will acquire chargeswhich will 3G correspond `to the positive orthenegative dif- Y ferentialvalues, as the case may be.- `The cathode ray pencils issuing from thecathodes 5l) andA 5I, by the aid of the deiiection fields set up by thesawtooth wave generator" 58 are sequentially 35 swept over thefstoring`surfaces 32 and 33. "By

' virtue of the device 59 the beam Scanning thestoring surface 33 willbe blocked out while the storing surface 32 is swept, and conversely,during v the scanning'of the storing surface 33the cathodeg\f40 ray`beam sweeping surface '32 will be blocked out. While these scanningactions are kproceeding the RF 4transmitter 51 will be acted uponconsecuv tively'by the 'positive and the negative brightness Dwhichbusbars 64-,66 are kconnected which correspond't'o the various lines ofthe picture reecreatorlor projection surface 61, while through leaddevices :69, 18 which are co-ordinated to the 55 verticalrows ofthepicture re-creating means on'picturefsurface 61.v Twocorrespondingcontacts of the said switches-69 and 10, say, contacts 1I and 12,'areeach connectedwthrough a tube 14,',60

jection lsurface. 61, the outputs of thesaid .two tubes having a highinternal vresistance.being interlc'onnected. 'I 'he picture r'e`creatingmeans. .65

While mechanical switches 63,669 and 10 have rays switch devices ofsuitable design.

The arrangement shown in Fig. 2' operates in themanner that While theswitch arm or wiper 63 is positioned on busbar 64, and while the cathoderay beam scans, in the storing device 30 of Fig. 1, for instance, thepositive differential values and then causes the same to be transmitted,the switch 69 of Fig. 2, transiently connects in sequence all cathoderay lamps which are connected with the busbar 64. Incidentally, thecondensers 19, 80 of those cathode ray lamps will be further charged forwhich the respective picture elements exhibit an increase in brightnesscompared with the preceding frame, which means that positive values ofthe differential picture result in a charge of the condensers at thecathode ray lamps. After a given contact element of rswitch 69 has beenabandoned, the charge upcn the corresponding condenser is preserved sothat thev luminescent screen of the cathode-ray lamp will be struck by aconstant cathode ray current throughout the entire time interval untilthe recurrence of transmission of a possibly altered `differential valuefor the picture element in question. After the positive differentialvalues have thus been transmitted, a motion of the switch 10 willinitiate transmission of the negative differential values in a similarmanner. Suppose that while the scanning beam in the storing device 3I ofthe transmitter sweeps the storing surface, and when the contact 12 ofswitch 10 is contacted, a negative differential value, for example, istransmitted so that through the amplifier 14 a discharge of thecondenser 19 will be ybrought about and that the other potential at thecontrol grid of the cathode ray lamp 11 is reduced. After the contactstep 12 has been abandoned, for the full interval of ytime until thetransmission of the next differential value, the luminescent screen ofthis cathode ray lamp will be impinged upon by a constant cathode raybeam. y

As already pointed out, particularly the switches l69 and 10, butoptionally also the switch 63 could be of the cathode ray type it beingrecommendable in this connection to so dispose the contacts in theswitch devices 69 and 10 that both switch pencils, by the agency of oneand the same deflectoreld, may be moved in synchronism and in phase,such that the pencil in switch device 1U will'reach its contacts onlyafter the pencil in switch device 69 has already swept its contacts.

' This condition, in fact, has been indicated also in the case of themechanical switches in Fig. 2 in that the angular path of both wiperscomprise the same angle a., and that in the switch 69 the contact stepsare disposed on the first half of the stroke, while in switch 1I) theyare arranged on the second half of the stroke. In a similar mannerarrangements may be made so that also in the storing means 30, 3|, thestoring surfaces 32 y and 33, respectively, will be scannedsequentially'.

are of a substantially lower frequency than the picture frequenciesheretofore customary in teleswitch 63 will be fairly low.

Another chance to carry the basic idea of the invention into practice,as already pointed out above, is that the scanning of the storingsurfaces 32, 33 at the sending end is effected by what has been calledas the velocity modulation method. For this purpose, the arrangementshown in Fig. 1 need tofbe altered but little, as illustrated in Fig. 3.The amplifiers 55, 56, which furnish the positive and the negativebrightness or intensity values of the differential picture in a way asknown from the velocity modulation method, act upon the sawtooth wavegenerators 58', 58; the potentials produced by them are applied, on theone hand, upon the deflector plate pairs 46, 41, while, on the otherhand, they are fed to the transmitter 51. The scanning pencils, aspreviously stated, rapidly sweep over those portions of the storingsurfaces 32, 33 where the f differential picture has no finiteamplitude, whereas a slower scanning takes place only where thedifferential picture exhibits contrast of detail. In this manner areduction in the requisite frequency band as above stated is realized,for

Avision 'work; hence, the speed of rotation of it will be understoodthat at all points along a line where the differential picture is ofzero amplitude the scanning beam will slip over quickly so that noappreciable time for transmission is` required.

In case the velocity modulation method is employed at the sending end,the receiver apparatus maybe built fundamentally similarly as describedby reference to Fig. 2. All that is required is that the switch arms orwipers of switch devices 69 and 16 must be caused to revolve at a rateof speed that will vary in accordance with the velocity modulation orscanning used at the sending end, rather than rotating the same at aconstant rate of speed. Switch 63 which should in this case v in theswitch 10.

As can be inferred from what precedes, re-

creator means have been assumed to exist at the f receiving endaccording to the present invention for the various picture elementswhich, after having been once excited, exhibit a constant after-effect.-In the exemplified embodiment as shown in Fig. 2 this is produced bythat the condenser coats 19, 80, retain their charges set u-p orsupplemented through the amplifiers 13, 14. In order to compensate forsuch gradual leaking away of these charges as may happen across theinsulation resistance of these condensers, and in order to effectoriginally the adjustment of the picture, one may proceed in such a waythat,

from time to time, the frame I2 of the film strip f is completely maskedat the sending end, while over the picture field of frame I3 a shutteris made to travel which will expose, for instance, only a few lines ofthe latter picture eld. Thus the absolute values of the picture pointbrightness or shading of the latter picture section (which, because ofthe complete masking of the lm frame I2 are the same as the differentialvalues) will be transrriitted and be recreated at the receiving end.Thereafter, for a comparatively long period of time, only thedifferential values in the brightness distribution of the picture can betransmitted, provided that there occursno complete change of scene inthe picture to be transmitted to a distant point. While the majorportion of the television picture is transmitted only in so far as thedifferential values compared with the preceding frame is concerned, itwould also be feasible to transmit for a few picture lines always theabsolute values of the picture element brightness and to changeperiodically the position of these picture lines. With this end in View,a few picture lines or strips inside the picture eld of frame i2 wouldhave to be completely masked so that inside the picture field of framei3 the absolute values rather than the differential values of thepicture element brightness in the masked lines would be televised.

lvVhat I claim is:

l. A system for transmitting optical images by television comprising atransmitter for sequentially transmitting a differential value o-isimultaneously developed picture signals representing correspondingelemental areas on at least two complete picture representations, areceiver wherein each picturel point representation may be pre-set to anoptical value representative of the optical value of the transmittedpicture for one frame thereof, and means for sequentially changing theoptical Values at the receiver in accordance with the transmitteddifferential representation. l

2. A television system comprising a transmitter, said transmitterincluding means for sequentially developing a differentialrepresentation of the optical values of each elemental area of at leastfg two complete optical representations to be transrepresenting means torepresent definite optical valuations, and means forsequentiallychanging said picture point representations in accordancewith the transmitted diierential representation senting means at saidpredetermined value for a predeterminable length of time, andA means forsequentially changing the operating level of said picture pointreproducing means in accordance with the transmitted differentialrepresentation.

4.' Apparatus in accordance With claim 2 wherein said receiver meanscomprises a plurality of light sources each representative of thepicturev point of the optical image to be reproduced, a

vacuum tube means having each of said picture point reproducing means inthe output circuit thereof, condenser means connected in the inputcircuit of each of said Vacuum tubes for maintaining tne tube at apredeterminable operating level, and commutating means adapted to enerigize said condenser means and to control the charge on said condensermeans in accordance with the transmitted diierential representations. Y

5. Method of television comprising the stepsy of simultaneouslyexamining at least two framesof the image to be transmitted, developinga differ-l ential representation of each of the'corresponding opticalpoints on both of said frames, transmit ting saiddevelopedrepresentations, and contro-lling the optical image reproduction inaccordance with the developed diiferential representations.

